Thermal voltage responsive relay



THERMAL COEFFICIENT {RELATIVELY SMALL July 29, 1941. TL RICH ETAL2,251,004

THERMAL VOLTAGE RESPQNSIVE RELAY Filed Nov. 29, 1938 Figlf I jOURCE BANDWIDTH ADJUSTMENT F EXPANSION RELATIVELY LARGE TEMPERATURE COEFFICIENTPOSITIVE OF RES|TANCE ZERO DROP OUT VOLTAGE ADJUSTMENT Fig.2. I

'TIHE IN SECONDS CHANGE IN VOLTAGE NECESSARY TO OPERATE CONTACTS ACTUALCHANGE 'V VOLTAGE Their Attorney.

Patented July 29, 1941 THERMAL VOLTAGE RESPONSIVE RELAY Theodore A.Rich, Schenectady, N. Y., and Lewis B. Runaldue, Pittsfield, Mass,assignors to General Electric Company, a corporation of New YorkApplication November 29, 1938, Serial No. 243,033

4 Claims.

This invention relates to control systems and more particularly toimprovements in control systems for automatic voltage regulators.

An object of the invention is to provide a novel and simple primaryvoltage sensitive control relay.

Another object of the invention is to provide a thermal voltagesensitive regulator control element having an inherent time delay in itsoperation.

A further object of the invention is to provide a thermal voltagesensitive relay having means for adjusting its drop-out voltage and itspick-up voltage.

An additional object of the invention is to provide a novel and simplethermal voltage sensitive relay which is compensated for variations inambient temperature.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawing and its scope will bepointed out in the appended claims.

In the drawing Fig. 1 is a diagrammatic illustration of the inventionapplied to an automatic one-step voltage booster, Fig. 2 is a sectionalView of the voltage relay taken on the line 2--2 of Fig. 1, and Fig. 3is a curve illustrating the time delay characteristic of the thermalvoltage responsive device.

Referring now to Fig. 1, there is shown therein a main alternatingcurrent supply circuit I connected to an alternating current loadcircuit 2 through a step-up autotransformer having a common winding 3and a. series voltage boosting winding 4. A solenoid operated contactor5 is arranged selectively to by-pass the winding 4 or connect it inseries in the load circuit. In order to prevent interruption of thecurrent in the load circuit during operation of the contactor 5, aby-pass 6 is provided for carrying the load current during a positionalchange of the contactor 5.

The operation of the contactor 5 in response to the voltage of thecircuit I is controlled by a voltage sensitive device I. Both thecontactor 5 and the device I are energized in accordance with thevoltage of the circuit I by means of a tertiary winding 8 on the voltageregulating transformer.

The voltage sensitive device I comprises a main supporting member 9which is preferably constructed of metal and which is shown by way ofexample as being in the form of a tube or cylinder. Mounted at one endof the tube is a fixed contact I0 and a movable co-operating,

contact II carried by an arm 12 on a curved contact operating member I3.Member I3 is supported by means of a spring blade I4, one end of whichis fastened to a suitable support I5 in the cylinder and the other endof which is fastened by any suitable means, such as by rivet I6, to themember I3. The spring blade I4 is straight in its natural or unflexedposition. However, it is forced into a bent position con forming to thecurvature of the middle portion of the member I3 by means of tensionstrips I! which are fastened to the member I3 by any suitable means,such as rivets I8, and which pass through the cylinder and are fastenedat the other end thereof to a yoke member I9. A

suitable linkage 20 is provided for adjusting or setting the initialtension of the strips I I. These strips are made of metal having arelatively large thermal coefficient of expansion and they are connectedby means of conductors 2| to the secondary winding of an adjustableratio transformer 22 whose primary winding is connected to be energizedby the tertiary winding 8. A substantially zero temperature coefficientof resistance resistor 23 is connected in series with the heating stripsI! for providing ambient temperature compensation in a manner which willbe described more fully hereinafter. The contacts I0 and II serve tocontrol the connection of the winding of the solenoid operating magnetof the contactor 5 to the tertiary winding 8.

The voltage ratio of the transformer 22 is made adjustable by means of atap switch 24 in order to adjust the drop-out voltage of the relay l.The band-width or pick-up voltage adjustment of the relay 1 is made bymeans of an adjustable stop member 25 for limiting the motion of themovable contact II away from the fixed contact I0.

In order to protect the device and prevent objectionable circulation ofair through the tubular member 9,'removable end caps 26 are providedtherefor.

In order to prevent injury to the operating winding of the contactor 5if the voltage of circuit I falls to such a low value as to be incapableof causing the contactor to pick up, a protective device 21 is connectedin the energizing circuit of the contactor 5. This device is shown asconsisting of a bimetallic switch arm 28 connected in series with aheater coil 29 and its arrangement is such that if a current above apredetermined value flows through the heater 29, the bimetallic switch28 will open the-energizing circuit of the contactor 5. 1

A manual control switch 30 is provided for selectively opening andclosing the regulator control circuit so that the regulator may readilybe taken out of service if desired.

The general operation of Fig. 1 is such that if the voltage of thesupply circuit I falls below a predetermined value, the contacts In andII will engage each other, thereby energizing the contactor and causingit to connect the boosting series winding 4 in the load circuit.Conversely, if the voltage of the circuit I rises above a predeterminedvalue, contacts IO and II will separate, thereby de-energizing thecontactor 5 and causing it to open the circuit through the serieswinding 4 and complete a by-pass connection around this winding.

The mechanism comprising the curved member I3 together with the bladespring I4 and the tension members I1 is the sole invention of TheodoreA. Rich and is described and claimed broadly in his application SerialNo. 243,032, filed November 29, 1938, and assigned to the assignee ofthe present application. This mechanism causes the contacts to operatewith a snap action. This snap action is the result of having the springblade I4 and the tension members I'l conform to surfaces of the memberI3 having difierent curvatures. These different curvatures are such thatas the member I3 rotates in a clockwise direction, as viewed in thedrawing, about a movable axis which corresponds approximately with theline where the spring I4 leaves engagement with the member I3, thevertical distance between the strips I7 and the spring I4 increases.

A contraction of the heating strips I1 due to cooling will exert atorque on the member l3. The lever arm of this torque is approximatelythe vertical distance between the heating strips I1 and the spring I4.This torque is opposed by the bending torque of the spring I4. When theheating strips have contracted to the point where the force exerted bythem times the lever arm of this force is greater than the moment of thespring, the member I3 is rotated clockwise. By reason of the curvaturesof the surfaces of the member I3 the lever arm increases rapidly enoughto maintain the torque of the heating strips I'I greater than theincreasing torque of the opposing spring strip I 4, although the forceexerted by the heating strips I1 decreases. Thus the contacts I0 and IIare snapped together. Upon heating of the strips Il they expand and asthe lever arm decreases rapidly when the member I3 turnscounterclockwise, the contacts I0 and II will be snapped open.

The heating of the strips I1 is controlled by the heating efiect of thecurrent passed through them. By means of the adjusting mechanism 20 theinitial tension of the members I! is adjusted to the proper value.Thereafter, the voltage at which the contacts separate is controlled byadjusting the ratio of the transformer 22 by means of the adjustingmember 24,

The proper adjustment or setting of the device 7 is obtained by firstsetting the adjusting means 24 on the tap which will cause the contactsI0 and II to separate when the voltage of circuit I reaches apredetermined value. The next step is to adjust the member 25 to thepoint at which contacts I0 and II close when the voltage falls apredetermined amount. The first adjustment is usually referred to as adrop-out voltage adjust ment and the last adjustment is usually referredto as' a bandwidth adjustment. Band-width may be defined as thedifierence between the voltage at which the contacts close and thevoltage at which the contacts open expressed as a open.

As the strips I! do not heat and cool or expand and contractinstantaneously with changes in main circuit voltage, the device I has atime delay characteristic. An example is shown in Fig. 3. This timedelay characteristic is inverse in that the larger the actual change involtage with respect to the change in voltage necessary to causeoperation of the contacts the shorter will be the time delay. Such aninverse characteristic is often desirable in automatic regulators as itprevents immediate response of the regulator to relatively small changesin voltage which often will correct themselves in a short time while itpermits substantially immediate response to relatively large changes involtage which ordinarily should be corrected immediately.

The curve of Fig. 3 can be used to determine the length of timenecessary for the contacts to operate for a given voltage change. Thevoltage before the change occurs should remain nearly constant for abouttwo minutes if the curve'is to be strictly correct. Use of the curve canmost easily be explained by an example. Assume that the drop-out voltageis adjusted for volts and that the voltage applied to the control devicehas been 108 volts for two minutes. Assume now that the voltage issuddenly increased to 112 volts. Since the contacts are adjusted tooperate at 110 volts the voltage change (from 108 to 110) necessaryeventually to operate the contacts is fifty per cent of the actualchange (from 108 to 112). From the curve it will be seen that a fiftyper cent voltage change corresponds to a time delay of eleven andone-half seconds.

In order that the setting of the device I will remain undisturbed withchanges in ambient temperature the tubular member 9 is made of amaterial having a thermal coeflicient of expansion which is somewhatless than that of the heating strips I1 and in addition thesubstantially zero temperature coefficient of resistance resistor 23 isconnected in series with the heating strips I1. The purpose of theresistor 23 is to reduce the overall temperature coefiicient of thecircuit containing the heating strips I! to a relatively low value.Theoretically, if the circuit containing the heating strips I! did notchange its resistance with changes in temperature, complete ambienttemperature compensation would be secured if the thermal coefficient ofexpansion of the supporting member 9 were the same as that of the stripsI! because then both of these elements would expand and contract byequal amounts for the same change in ambient temperature and hence therewould be no ambient temperature produced differential expansion tendingto cause operation of the contacts. However, changes in ambienttemperature actually cause changes in the resistance of the heatingstrips I1, thereby causing changes in heating current to flow therein.For example, if the ambient temperature falls, the heating strips I!tend to contract and also their electrical resistance decreases. Thismeans that for a given voltage applied to the transformer 22 the currentin the heating strips will increase and therefore the heating effectofthe current in these strips will increase. Consequently, thetemperature rise above ambient of the heating strips is not constantwith variations in ambient temperature. By making the coeflicient ofexpansion of the supporting means less than the coeflicient of expansionof the strips I1 the heating strips will tend to contract enough morethan the support for lower ambient temperatures to compensate for thefact that their temperature rise for the same voltage is greater. Thenet result is that the tension in the heating strips is the same,

At the present time it is believed that a stainless steel known asCarpenter No. 2 cutlery steel is best for the heating strips. This steelcontains chromium, nickel and carbon. It can be hardened. In thehardened state and at about 70 C. it has a coefiicient of expansion of11x10- inches per inch per degree centigrade. The most suitable materialknown at the present time for the support 9 is a stainless steelcontaining 12 to 15 per cent chromium and 0.12 per cent carbon. Thismaterial has a coefficient of expansion of l0.53 10 inches per inch perdegree centigrade.

Should the voltage of the line I drop approximately '75 per cent belownormal the contacts 10 and H wouldof course close, thus energizing thesolenoid but this device would not pick up its armature. With thearmature in the dropped-but position the solenoid has a relatively lowreactance and thus a relatively high current would flow through thesolenoid winding. It has been found that if this condition persists fora long enough time the contactor 5 will be injured.

In order to protect the winding 5 against this injury the protectivedevice 21 is arranged to open its contacts and thus reduce the currentin the winding 5. As soon as the protective device 21 cools, itscontacts will again close thereby causing relatively high current toflow in the winding 5 but this will .again cause the device 21 to beheated so that this device will alternately open and close its contacts,thus maintaining the entire system in a condition where it willautomatically operate properly as soon as the voltage of circuit lreturns to normal, while,

at the same time it prevents serious over-heating of the contactor 5.

While there has been shown and described a particular embodiment of thisinvention, it will be obvious to those skilled in the art that variouschanges and modifications can be made therein without departing from theinvention and, therefore, it is aimed in the appended claims to coverall such changes and modifications as fall within the true spirit andscope of the invention.

What we claim as new and desire to secure by Letters Patent of theUnited States, is:

1. In a thermal relay, a. supporting member, a switch, said switchhaving a tensioned operating member which snaps the switch contacts openand closed when the tension thereon changes a predetermined amount, athermally expandible electrical conducting member having one end thereoffastened to said supporting member and having the other end thereoffastened to said operating member, said operating member being whollysupportedby its tensioning means including said expandible member, andan electric circuit for passing a heating current through saidexpandible member.

2. In a thermal voltage regulating relay, a supporting member, a switch,said switch having a tensioned operating member which snaps the switchcontacts open and closed when the tension thereon changes apredetermined amount, a thermally expandible electrical conductingmember having one end thereof fastened to said supporting member andhaving the other end thereof fastened to said operating member, anelectric circuit for passing a heating current through said expandiblemember, means for adjusting the drop-out voltage of said relaycomprising a variable ratio transformer for supplying current to theheating circuit for said expandible member, and means for adjusting theband-width of said relay comprising an adjustable stop for limiting theseparation of said contacts.

3. In an ambient temperature compensated thermal relay, a pair ofcontacts, a pair 01' thermally expandible members arranged so that adifierence in thermal expansion and contraction thereof will operatesaid contacts, and a circuit having a positive temperature coeflicientof resistance for heating one of said members in accordance with thevoltage of an electric circuit, the other of said members having a lowercoeflicient of thermal expansion than said heated member so as tocompensate for the temperature coefiicient of resistance of said heatingcircuit.

4. In an ambient temperature compensated thermal relay, a pair ofcontacts, a pair of thermally expandible members arranged so that adifference in thermal expansion thereof will operate said contacts, acircuit having a positive temperature coefficient of resistance forheating one of said members in accordance with the voltage of an electrccircuit, the other of said members having a lower coefiicient of thermalexpansion than said heated member so as to compensate for thetemperature coeflicient of resistance of said heating circuit, and asubstantially zero temperature coeflicient resistor connected in saidheating circuit for reducing the overall temperature coefficient ofresistance of said circuit.

THEODORE A. RICH. LEWIS R. RUNALDU'E.

